As a technique of stabilizing liquid crystal alignment using a polymer, a technique is disclosed in which a photocurable monomer is added to a twisted nematic liquid crystal, followed by light irradiation in an attempt in polymer-stabilization of alignment (refer to Non-Patent Document 1). The nematic liquid crystal described in the relevant cited document retains an original twisted structure before polymer stabilization is performed even after photocuring, and tries not to eliminate liquid crystallinity and alignment properties of a liquid crystal material for display by using a liquid crystalline monomer containing a liquid crystal skeleton as a photocurable monomer.
The relevant cited document discloses an example of a polymer-stabilized liquid crystal device produced by adding a photocurable monomer into a twisted nematic liquid crystal host in a concentration of several %, followed by light irradiation in a state where no voltage is applied. Furthermore, Examples disclose voltage-dielectric constant characteristics when the concentration of the liquid crystalline monomer is varied (2%, 3%, 4%, 5%). As is apparent from the results, a saturated dielectric constant decreases when the addition amount of the liquid crystalline monomer increases so as to have a higher voltage applied for saturation, and thus a driving voltage increases, resulting in an increased driving voltage of the device. This is a point of issue in practical use of a polymer-stabilized liquid crystal device.
When a smectic liquid crystal is used for the liquid crystal device, it exhibits behavior which is basically different from that of a nematic phase. When the smectic phase is applied for the liquid crystal display device, we utilize a phenomenon that a major axis of liquid crystal molecules (direction of molecules) inclines from a direction where a layer structure of a smectic phase is formed. An angle at which liquid crystal molecules incline from a layer normal line direction is called as a tilt angle, which is a characteristic physical property when the smectic liquid crystal is used for the liquid crystal device, and acquisition of the tilt angle suited for the liquid crystal device is desirable. However, the cited document does not disclose an influence on this tilt angle.
As an example of applying a polymer-stabilized twisted nematic liquid crystal for a TFT liquid crystal display device, a technique is disclosed in which a small amount of a photocurable liquid crystalline monomer is added and polymerized in a TFT liquid crystal display device, thereby stabilizing alignment by an action of the formed polymer (refer to Patent Document 1). However, in the relevant cited document, a response speed of a liquid crystal upon falling is revised, but aforementioned problem of the driving voltage is not solved. Also, the cited document does not disclose an influence that addition of the photocurable liquid crystal monomer exerts on aforementioned tilt angle.
In a polymer-stabilized liquid crystal display device, a technique of applying for an OCB (optically compensated birefringence) mode is proposed, except for a twisted nematic liquid crystal display mode (refer to Non-Patent Document 2). The relevant cited document discloses a technique of dispersing liquid crystal molecules in a photoreactive monomer, applying an external electric field to obtain a desired bend alignment structure, and immobilizing the bend alignment by light irradiation. In the OCB mode, liquid crystal molecules take spray alignment in an initial state by applying a high voltage when a display device is booted. Since it requires an alignment transition operation for converting it into bend alignment, the bend alignment is preliminarily immobilized with a polymer so as to enable display boot without requiring an alignment transition operation to this bend alignment, as well as display by the OCB mode, and thus overcoming drawbacks of the OCB mode.
There is also a technique disclosed in which liquid crystal molecules are immobilized in a polymer liquid crystal matrix on a sheet of substrate, thereby immobilizing hybrid alignment, and two sheets of substrate are laminated so as to contact the homogeneous alignment portion on the substrate to make bent alignment of an OCB mode, which is used for a liquid crystal display device (refer to Patent Document 2). This case is characterized by forming bent alignment without applying a voltage.
However, aforementioned problem of the driving voltage is not solved in these inventions disclosed in the cited documents, and also the cited documents do not disclose an influence of the addition of the photocurable liquid crystal monomer on aforementioned tilt angle.
With respect to a polymer-stabilized ferroelectric liquid crystal (polymer-stabilized FLC), there is a proposed technique for making an attempt of polymer stabilization, in which a FLC material is used together with a monomer, and ultraviolet rays are irradiated while liquid crystal molecules are aligned in one direction by applying an eclectic field (refer to Non-Patent Documents 3 and 4).
There is also a polymer-stabilized ferroelectric liquid crystal display device disclosed, which is obtained by injecting a liquid crystal composition containing a ferroelectric liquid crystal and a monofunctional liquid crystalline (meth)acrylate monomer into a liquid crystal cell, and irradiating it with ultraviolet rays at a temperature at which the composition exhibits a predetermined liquid crystal phase, thereby polymerizing the monofunctional liquid crystalline (meth)acrylate monomer (refers to Patent Documents 3, 4, and 5). Although polymer stabilization for the alignment of liquid crystal molecules imparts a novel function, and aforementioned OCB mode and a device using a monofunctional liquid crystalline (meth)acrylate as a ferroelectric liquid crystal have such features as obtaining satisfactory bent alignment and enabling satisfactory grayscale display, there was a problem that the polymer obtained by polymerization of the monofunctional liquid crystalline (meth)acrylate monomer had unfavorable heat resistance, resulting in unsatisfactory reliability at a high temperature. Furthermore, there was a problem that the driving voltage was high in a device using a monofunctional liquid crystalline (meth)acrylate as a ferroelectric liquid crystal. There was also a problem of high driving voltage in a document disclosed, in which it becomes possible to achieve grayscale display in proportion to an applied voltage when polymer stabilization is conducted by ultraviolet exposure in a smectic A phase and phase transition is conducted by annealing to the smectic C phase (refer to Patent Document 3). As described above, aforementioned problem of the driving voltage is not solved in these inventions disclosed in the cited documents, and also the cited documents do not disclose an influence that the addition of the photocurable liquid crystal monomer exerts on aforementioned tilt angle.
There is a polymer-stabilized ferroelectric liquid crystal display device disclosed, using a polyfunctional liquid crystalline monomer which gives a polymer having superior heat resistance to a monofunctional liquid crystalline (meth)acrylate monomer (refer to Patent Document 6). However, many of the polyfunctional liquid crystalline monomers require a high temperature (80° C. or higher) at which liquid crystallinity is exhibited, and it becomes necessary to raise temperature in a stage before ultraviolet rays are irradiated for production of a polymer-stabilized liquid crystal device. As a result, there occurred a problem that undesirable thermopolymerization is induced, resulting in deterioration of uniformity of liquid crystal alignment.
Since the content of a liquid crystal is from 60 to 95% by weight and the remainder is a network-shaped polymer, light scattering commonly observed in polymer-dispersed liquid crystals arises. This scattering causes a decrease in contrast of a display device using polarized light. Therefore, the liquid crystal display device had a drawback that other means is required to improve the contrast. Furthermore, regarding a polyfunctional liquid crystalline (meth)acrylate, reliability of polymer stabilization increases as a result of suppression of thermal fluctuation of a mesogen group when compared with a monofunctional liquid crystalline (meth)acrylate. However, there was a problem that an interaction with a low-molecular liquid crystal increases and thus a driving voltage increases.    [Patent Document 1]    Japanese Unexamined Patent Application, First Publication No. 2005-10202    [Patent Document 2]    Japanese Unexamined Patent Application, First Publication No. 2003-248226    [Patent Document 3]    Japanese Unexamined Patent Application, First Publication No. Hei 9-211462    [Patent Document 4]    Japanese Unexamined Patent Application, First Publication No. Hei 9-211463    [Patent Document 5]    Japanese Unexamined Patent Application, First Publication No. Hei 11-21554    [Patent Document 6]    Japanese Unexamined Patent Application, First Publication No. Hei 6-194635    [Non-Patent Document 1]    Japan Society for Promotion of Science, Organic Material for Information Science, 142nd Committee, Sectional Meeting A (Liquid Crystal Material), Documents of 91st Society (pp. 28-30)    [Non-Patent Document 2]    The Institute of Electronics, Information and Communication Engineers Technical Research Report, Vol. 95, (EID95-17), pp. 43-48, 1995    [Non-Patent Document 3]    H. Fume, Jpn. J. Appl. Phys. 36, L1517 (1997)    [Non-Patent Document 4]    H. Fume, Jpn. J. Appl. Phys., 37, 3417 (1998)